1. Field of the Invention
The present invention relates to a wire electric discharge machine including an average discharge delay time calculating unit.
2. Description of the Related Art
In general, a wire electric discharge machine executes machining gap average voltage fixed servo feed control for obtaining a fixed discharge gap by controlling a voltage between a wire electrode and a workpiece (a machining gap voltage) such that an average of measured voltages (a machining gap average voltage) in a gap between the wire electrode and the workpiece (a machining gap) coincides with a target value.
In executing the machining gap average voltage fixed servo feed control, in general, the wire electric discharge machine calculates the machining gap average voltage by full-wave rectifying the machining gap voltage with a full-wave rectifier circuit and converting the machining gap voltage into a voltage waveform close to a direct current with a low-pass filter circuit. When the machining gap average voltage is calculated in this way, even if a machining gap voltage waveform is the same, an error occurs in a measurement value depending on a machine because of a measurement error of an analog circuit. Therefore, it is sometimes difficult to accurately reproduce machining. A voltage waveform appearing in the machining gap when electric discharge occurs has a wide variety of frequency components from several ten kilohertz to several ten megahertz. Therefore, an error occurs in a machining gap average voltage obtained from a measurement circuit because of errors in frequency characteristics of components included in the analog circuit and individual differences of specifications of the components.
To solve this problem, Japanese Patent Application Laid-open No. 50-1499 discloses a technique for detecting a no-load time (hereinafter referred to as “discharge delay time”) from application of a voltage to a machining gap until occurrence of electric discharge and performing servo feed such that the detected discharge delay time coincides with a predetermined time. In electric discharge machining, it is known that, when a voltage applied between the wire electrode and the workpiece is fixed, there is a correlation between the discharge delay time from the application of the voltage until the occurrence of the electric discharge and a gap amount between the wire electrode and the workpiece.
However, in actual machining, all discharges do not always occur in completely the same discharge delay time every time. The discharge delay time greatly fluctuates in every voltage application centering on a value obtained from the correlation between the discharge delay time and the gap amount. In particular, in a situation in which a large quantity of large conductive sludge floats in the machining gap such as rough machining or two-time machining, electric discharge sometimes occurs immediately after the voltage application. The discharge delay time in that case is a value much smaller than a value corresponding to an actual machining gap. As a result, a fed speed command to a servo greatly fluctuates in every voltage application, control becomes unstable, and, as a result of machining, it is extremely difficult to obtain a fixed machining groove width.
To solve this problem, Japanese Patent Application Laid-open No. 55-101333 discloses a technique for integrating a discharge delay time from voltage application until occurrence of electric discharge for a predetermined period, comparing an integrated value of the discharge delay time with an upper limit threshold and a lower limit threshold set in advance, and controlling a relative distance between a wire electrode and a workpiece according to a result of the comparison. Further, Japanese Patent Application Laid-open No. 2-109633 discloses a technique for totaling discharge delay times at every predetermined sampling cycle, applying a low-pass filter for a predetermined cutoff frequency to a total value of the discharge delay times to perform filter processing to remove a sudden change, and controlling a relative position between a wire electrode and a workpiece such that a value subjected to the filter processing coincides with a predetermined value.
In both of the two patent documents mentioned above, the discharge delay time in the predetermined period is integrated and the wire electrode is servo-fed with respect to the workpiece such that the integrated value coincides with the predetermined value. Therefore, since a wire electric discharge machine less easily responds to a sudden change in the discharge delay time due to fluctuation in the discharge delay time and electric discharge via the sludge, controllability is considered to be stabilized to a certain degree.
Japanese Patent Application Laid-open No. 7-246519 discloses a technique for detecting the number of times of electric discharge in every predetermined period, dividing a period of the detection by the detected number of times of electric discharge to thereby calculate a total time per electric discharge, and subtracting an energization time and a quiescent time set in advance from the total time to calculate an average discharge delay time.
On the other hand, there is also a technique for calculating, rather than a discharge delay time, a machining gap voltage corresponding to the discharge delay time and performing feed control. Japanese Patent Application Laid-open No. 2003-165030 discloses a method of measuring a machining gap average voltage including a quiescent time, preparing in advance, in a table, a correction value for calculating a machining gap average voltage (a corrected machining gap average voltage) excluding the quiescent time, based on a value of the machining gap average voltage and the set quiescent time, and calculating a machining gap average voltage not including the quiescent time, using a machining gap average voltage after correction obtained from the table.
Japanese Patent Application Laid-open No. 2-298426 discloses a method of dividing a sum TB of discharge delay times of a discharge pulse in a predetermined time (a measurement time TA) by the measurement time TA to calculate a time ratio (TB/TA) and multiplying the calculated time ratio by a reference voltage E set in advance to calculate an average voltage V (=(TB/TA)×E). Japanese Patent Application Laid-open No. 2004-136410 discloses a method of calculating an average voltage V according to an expression V={(Ta−N×(Ton+Toff))/Ta}×E from a machining gap voltage E, a quiescent time Toff during electric discharge, an energization time Ton during electric discharge, a measurement time Ta, and the number of times of electric discharge N measured in the measurement time Ta.
In the techniques disclosed in Japanese Patent Application Laid-open No. 50-1499 and Japanese Patent Application Laid-open No. 55-101333, the discharge delay time is integrated at every predetermined sampling frequency or in the predetermined period. Therefore, a measurement period of the discharge delay time is fixed. Moreover, the number of times of voltage application is not taken into account in the measurement of the discharge delay time.
In wire electric discharge machining, an auxiliary power supply for inducing electric discharge is connected to a machining gap. When electric discharge by the auxiliary power supply is detected, a main power supply is connected to the machining gap to apply a machining current. After the application of the machining current, for prevention of disconnection, in general, a quiescent time during which a voltage is not applied is inserted. If the machining is stable, the electric discharge occurs at a substantially fixed rate and the quiescent time is inserted at a substantially fixed rate. However, when a flow of machining liquid changes in a step portion of workpiece and discharge of sludge is deteriorated or a facing area of a wire electrode and the workpiece suddenly changes in a corner portion of the workpiece and an electric discharge frequency changes, the number of times of electric discharge in the fixed measurement time changes and a rate of the quiescent time in the measurement period greatly changes. Therefore, the number of times of voltage application included in the measurement time greatly increases and decreases.
If the machining is stable and a gap amount of the machining gap is fixed, the discharge delay time is also fixed. In a state of the stable machining, if the number of times of voltage application in the predetermined measurement period is large, an integrated value of the discharge delay time must be large. If the number of times of voltage application is small, the integrated value must be small. In this way, as a result of the change in the electric discharge frequency, when it is attempted to fix the integrated value of the discharge delay time irrespective of the fact that the number of times of voltage application in the predetermined measurement period changes, the gap amount of the machining gap cannot be controlled to be fixed.
Further, when an amount and a direction of a bend of a wire change and a machining gap average voltage suddenly drops because of, for example, the change in the flow of the machining liquid and it is determined that a machining gap state is close to a short circuit, the quiescent time may suddenly be increased. When a machining amount suddenly changes in the corner portion or a cutting start portion, to reduce the electric discharge frequency to stabilize the machining, the quiescent time may be sometimes greatly extended.
As the quiescent time to be inserted in such a case, a long time of several hundred microseconds to several milliseconds is sometimes necessary. In Japanese Patent Application Laid-open No. 55-101333 and Japanese Patent Application Laid-open No. 2-109633, as explained above, the measurement time of the discharge delay time is fixed and, moreover, the number of times of voltage application during the measurement period is not taken into account at all. Therefore, the integrated value of the measured discharge delay time is a value having no correlation with the gap amount between the workpiece and the wire electrode.
When such a long quiescent time is inserted, the measurement time is sometimes completely included in the inserted quiescent time. In this case, integration of the discharge delay time (a no-load time from application of a voltage to the machining gap until occurrence of an electric discharge) is zero. Therefore, an actual machining gap state is not reflected at all. Further, when discharge delay time fixed control is performed in a state in which the integration of the discharge delay time is zero, relative speed between the wire electrode and the workpiece becomes extremely low and the discharge delay time becomes long. Therefore, as a result of the machining, a fixed machining groove cannot be obtained.
In the case of a general wire discharge machining method by machining gap average voltage fixed servo feed control for controlling a machining gap average voltage subjected to averaging processing by a full-wave rectifier circuit or a low-pass filter circuit to be fixed, when a large quiescent time is inserted for the purpose of reducing an electric discharge frequency and reducing a machining amount, an average voltage drops and machining speed decreases. For example, in finishing an outer corner portion of a workpiece, when control is performed to insert a long quiescent time for the purpose of reducing a machining amount, taking into account a decrease in a necessary machining amount, if a shape having many corners is machined, machining speed is extremely low at each of the corners. Therefore, a machining time is extremely long.
In both of the techniques for calculating a machining gap voltage disclosed in Japanese Patent Application Laid-open No. 2-298426 and Japanese Patent Application Laid-open No. 2004-136410, the rate of the discharge delay time in the predetermined measurement period is calculated and the rate is multiplied by the reference voltage to obtain the average voltage. That is, the rate of the integrated value of the discharge delay time to the predetermined measurement time is measured to approximately calculate the machining gap average voltage calculated by the conventional analog circuit. Therefore, the average voltage is fundamentally different from a machining gap voltage based on an accurate average discharge delay time that is to be calculated in the present invention. For example, when the quiescent time is extended, naturally, the rate of the integrated value of the discharge delay time to the predetermined measurement time decreases and the calculated average voltage also decreases. In this way, the calculated average voltage is greatly affected by the quiescent time and the number of times of voltage application is not taken into account. Therefore, the average time has no correlation with an accurate average discharge delay time having a correlation with a gap amount of a machining gap.